This invention relates in general to wireless communication systems, and more specifically to a method and apparatus in a wireless receiver for demodulating a continuous-phase frequency-shift-keyed signal received in a delay spread environment including noise.
For additive white Gaussian noise (AWGN) channels, an optimum receiver for an M-ary continuous-phase frequency shift keyed (CPFSK) signal consists of a bank of M matched filters followed by a sequence estimator. For noncoherent receivers, the sequence estimator may be implemented as a multiple-symbol matched filter (MSMF). The MSMF receiver combines outputs from the single-symbol matched filter (SSMF) to achieve optimum AWGN sensitivity. However, delay spread caused by multipath and/or simulcast transmission causes phase discontinuities in the received signal, which can cause the performance of the MSMF to be unacceptable. In such environments, the SSMF still performs well, and is the preferable demodulation technique. Thus, we have a situation in which the MSMF outperforms the SSMF in AWGN channels, while the SSMF outperforms the MSMF in delay spread channels.
What is needed is a method and apparatus in which the receiver can detect the presence of such channel conditions, and choose whether to use single-symbol (SSMF) outputs, or to use the results obtained by combining multiple outputs from the single-symbol correlators (MSMF). Preferably the method and apparatus will not need any feedback from the forward error correction decoder, i.e., the method and apparatus will operate quickly and efficiently on the physical layer.
An aspect of the present invention is a method in a wireless receiver for demodulating a continuous-phase frequency-shift-keyed signal received in a delay spread environment including noise. The method comprises the steps of demodulating a block of symbols of the signal through a single-symbol matched-filter technique, thereby generating a first set of data; and demodulating the block of symbols through a multi-symbol matched-filter technique, thereby generating a second set of data. The method further comprises the steps of measuring a characteristic of the signal, thereby obtaining a measurement; and choosing one of the first and second sets of data for output, based upon the measurement.
Another aspect of the present invention is a demodulator in a wireless receiver for demodulating a continuous-phase frequency-shift-keyed signal received in a delay spread environment including noise. The demodulator comprises an input interface for receiving the signal, and a processor coupled to the input interface for processing the signal. The demodulator further comprises an output interface for outputting demodulated data. The processor is programmed to demodulate a block of symbols of the signal through a single-symbol matched-filter technique, thereby generating a first set of data; and to demodulate the block of symbols through a multi-symbol matched-filter technique, thereby generating a second set of data. The processor is further programmed to measure a characteristic of the signal, thereby obtaining a measurement; and to choose one of the first and second sets of data for output, based upon said measurement.
Another aspect of the present invention is a wireless receiver for receiving and demodulating a continuous-phase frequency-shift-keyed signal received in a delay spread environment including noise. The wireless receiver comprises a receiver front end for receiving and down-converting the signal to derive a down-converted signal; and a demodulator coupled to the receiver front end for demodulating the down-converted signal. The wireless receiver further comprises a user interface coupled to the demodulator for interfacing with a user. The demodulator comprises an input interface for receiving the down-converted signal, and a processor coupled to the input interface for processing the down-converted signal. The demodulator further comprises an output interface for outputting demodulated data. The processor is programmed to demodulate a block of symbols of the down-converted signal through a single-symbol matched-filter technique, thereby generating a first set of data; and to demodulate the block of symbols through a multi-symbol matched-filter technique, thereby generating a second set of data. The processor is further programmed to measure a characteristic of the down-converted signal, thereby obtaining a measurement; and to choose one of the first and second sets of data for output, based upon said measurement.